1. Field of the Invention
The present invention relates to a high gloss printing sheet. More particularly, the present invention relates to a printing sheet provided with an electron beam-cured resin coating layer, arranged on the outermost surface of the sheet, having an enhanced compatibility with printing ink and exhibiting a high gloss before and after printing.
2. Description of the Related Art
It is known that a cast coated paper sheet having a high gloss is produced by coating an aqueous coating liquid containing, as principal components, a pigment and a binder on a front surface of a substrate paper sheet and the coated liquid layer is pressed onto a heating mirror surface of a casting base while the coated liquid layer still contains water and exhibits a plasticity, and then dried to form a high gloss front coating layer. The conventional cast coated paper sheet is, however, not satisfactory in gloss and thus a new type of high gloss sheet is strongly demanded.
Also, when another coating layer is further formed on the back surface of the substrate sheet in the same conventional cast-coating method as mentioned above, since the back coating layer surface comes into contact with the heating mirror surface so as to rapidly evaporate away water from the back coating layer, the front coating layer is swelled and softened by the water vapor. The phenomenon usually causes the surface smoothness and gloss of the front coating layer to decrease and thus the clearness of ink images printed on the front coating layer surface to deteriorate.
The above-mentioned disadvantages of the conventional printing sheet can be solved by a laminate coating method in which a thermoplastic resin melt is formed into a filmy stream, the filmy resin melt stream is coated on a front surface of a substrate sheet, the resultant resin melt layer of the laminate is brought into contact with a mirror-casting face of a rotating metal drum under pressure and then cooled on the mirror-casting surface to solidify the resin layer and the resultant high gloss sheet is removed from the mirror-casting face.
Alternatively, a laminate sheet having a high gloss coating layer can be produced by a cast-electron beam-irradiation method in which a coating liquid containing an electron beam-curable unsaturated organic material is coated on a front surface of a substrate sheet, the resultant coating layer is brought into contact with a casting face of a casting sheet or a casting drum under pressure and an electron beam is irradiated to the coating layer so as to cure the coating layer.
The cast coated sheets produced by the above-mentioned methods have a satisfactory high gloss.
Nevertheless, these conventional sheets are unsatisfactory as printing sheets. Namely, the conventional cast-coated sheets have an unsatisfactory suitability for printing, especially an insufficient compatibility with printing ink, in other words an unsatisfactory ink-setting property.
Particularly, when a conventional electron beam-curable unsaturated organic compound is coated on the substrate sheet and cured by electron beam irradiation, the resultant resin coating layer consists of a dense film which has a poor compatibility with the ink and thus does not allow the ink to penetrate into the resin coating layer. Also, the conventional electron beam-cured resin coating layer exhibits a poor ink-setting property. Therefore, when, on the printed surface of the conventional printing sheet, another sheet is superposed, the ink located on the printed surface is easily transferred to and soils the back surface of the superposed sheet.
This problem is considered very difficult to solve.
Also, it is known that even when the conventional electron beam-curable unsaturated compound can form a cured resin coating layer having a good ink-setting property, the gloss of the cured resin coating layer is degraded by a printing operation.
Accordingly, there is a strong demand of providing a new type of high gloss printing sheet free from the above-mentioned disadvantages.